SYPHON RECORDER. 



241 



place the needle on the axis of the coil or in its mean plane ; 

 3, To increase the distance d from the needle to the central coil. 



The needle and the coil support, moreover, two mirrors in which 

 an observer can, from a fixed position, view with two telescopes the 

 images of two divided scales, having also a key to reverse the current 

 so as to get rid of the remaining errors of adjustment in the usual 

 way. 



860. SYPHON RECORDER. Apparatus with movable coils may 

 acquire great sensitiveness if the intensity of the external field is 



Fig. 164. 



sufficient. Sir W. Thomson made use of this property in con- 

 structing his syphon recorder, which is used as a receiving instru- 

 ment in submarine telegraphy (Fig. 164). The coil s is wound 

 on a thin rectangular frame placed between the poles of an electro- 

 magnet, the ends of which A and B are marked on the figure. A 

 fixed mass of soft iron f occupies the space left in the centre of the 

 coil, and increases the strength of the field in the region which the 

 wire traverses. The coil is attached to a bifilar suspension. The 

 lower part carries a weight Q which slides along a board, which is 

 more or less inclined to the vertical and serves to regulate the 



VOL. II. R 



